Reductive Elimination From Tetra-Alkyl Cuprates [MeCu(CF)] (n=0-4): Beyond Simple Oxidation States.

In recent years, the electronic structures of organocuprates in general and the complex [Cu(CF)] in particular have attracted significant interest. A possible key indicator in this context is the reactivity of these species. Nonetheless, this aspect has received only limited attention.

The Fe-MAN Challenge: Ferrates-Microkinetic Assessment of Numerical Quantum Chemistry.

Organometallic species, such as organoferrate ions, are prototypical nucleophiles prone to reacting with a wide range of electrophiles, including proton donors. In solution, the operation of dynamic equilibria and the simultaneous presence of several organometallic species severely complicate the analysis of these fundamentally important reactions. This can be overcome by gas-phase experiments on mass-selected ions, which allow for the determination of the microscopic reactivity of the target species.

Intramolecular Phenyl Transfer from a Boronate to Lithium in the Gas Phase Reveals Crucial Role of Solvation in Transmetalations.

In contrast to its behavior in solution, the adduct [(LiBr)(Bu)(Ph)Bpin] (pin=pinacol) transfers its phenyl anion from boron to lithium upon fragmentation in the gas phase. Quantum chemical calculations predict this exceptional transmetalation to be exothermic relative to the separated reactants, [(Bu)(Ph)Bpin] and LiBr, which we attribute to the high phenyl-anion affinity of the coordinatively unsaturated LiBr unit. The addition of a single molecule of tetrahydrofuran drastically reduces the phenyl-anion affinity of LiBr and thereby renders the transmetalation from boron to lithium endothermic.

Gas-Phase Alkali-Metal Cation Affinities of Stabilized Enolates.

The chemistry of alkali-metal enolates is dominated by ion pairing. To improve our understanding of the intrinsic interactions between the alkali-metal cations and the enolate anions, we have applied Cooks' kinetic method to determine relative M (M=Li, Na, K) affinities of the stabilized enolates derived from acetylacetone, ethyl acetoacetate, diethyl malonate, ethyl cyanoacetate, 2-cyanoacetamide, and methyl malonate monoamide in the gas phase. Quantum chemical calculations support the experimental results and moreover afford insight into the structures of the alkali-metal enolate complexes.

Unimolecular Reactivity of [Cu(R)(CF ) ] Complexes (R=Organyl): Stepwise versus Concerted Mechanism in Copper-Mediated Trifluoromethylation.

The cuprate complexes [Cu(R)(CF ) ] (R=organyl) offer an efficient synthetic access to valuable trifluoromethylation products RCF . Here, electrospray-ionization mass spectrometry is used to analyze the formation of these intermediates in solution and probe their fragmentation pathways in the gas phase. Furthermore, the potential energy surfaces of these systems are explored by quantum chemical calculations.

C versus O Protonation in Zincate Anions: A Simple Gas-Phase Model for the Surprising Kinetic Stability of Organometallics.

For better understanding the intrinsic reactivity of organozinc reagents, we have examined the protolysis of the isolated zincate ions Et Zn , Et Zn(OH) , and Et Zn(OH) Li by 2,2,2-trifluoroethanol in the gas phase. The protonation of the hydroxy groups and the release of water proceed much more efficiently than the protonation of the ethyl groups and the liberation of ethane. Quantum-chemical computations and statistical-rate theory calculations fully reproduce the experimental findings and attribute the lower reactivity of the more basic ethyl moiety to higher intrinsic barriers, which override the thermodynamic preference for its protonation.

Live Monitoring of Anionic Living Polymerizations by Electrospray-Ionization Mass Spectrometry.

Anionic polymerizations are of exceptional practical importance, but difficult to analyze due to the high reactivity of the growing polymer chains. Here, we demonstrate that electrospray-ionization mass spectrometry (ESI-MS) permits direct observation of the active carbanionic intermediates formed in the anionic ring-opening polymerization of 1-cyanocyclopropanecarboxylate in tetrahydrofuran. This includes the identification of a side product, as well as real-time analysis of the polymerization reaction.

In-Situ Analysis of Anionic Coordination Polymerizations by Electrospray-Ionization Mass Spectrometry.

Anionic coordination polymerizations proceed via highly reactive intermediates, whose in situ analysis has remained difficult. Here, we show that electrospray-ionization mass spectrometry is a promising method to obtain detailed information on the polymerization process. Focusing on polymerization reactions of 1,3-dienes initiated by CoCl /RLi (R=Me, nBu, tBu, Ph), we directly observe the growing polymer chains and characterize the active anionic cobalt centers by gas-phase fragmentation experiments.

Microscopic Reactivity of Phenylferrate Ions toward Organyl Halides.

Despite its practical importance, organoiron chemistry remains poorly understood due to its mechanistic complexity. Here, we focus on the oxidative addition of organyl halides to phenylferrate anions in the gas phase. By mass-selecting individual phenylferrate anions, we can determine the effect of the oxidation state, the ligation, and the nuclearity of the iron complex on its reactions with a series of organyl halides RX.

Anionic Dimers of Fluorinated Alcohols.

Negative-ion mode electrospray ionization of solutions of ethanol (ROH), 2-fluoroethanol (ROH), 2,2-difluoroethanol (ROH), and/or 2,2,2-trifluoroethanol (ROH) produces anionic dimers of the types (RO)H and (RO)(RO)H. The exchange reactions of these anionic dimers with the neutral alcohols are examined in a quadrupole-ion trap to extract kinetic data, from which the reaction Gibbs energies are obtained. In all cases, the formation of anionic dimers containing the more highly fluorinated alcohols is favored.

Gas Phase Protolysis of Trisarylzincate Anions.

We have applied a combination of tandem-mass spectrometry, quantum-chemical calculations, and statistical rate theory computations to examine the gas phase reactions between the trisarylzincate anions ArZnPh (Ar = -X-CH, X = NMe, OMe, Me, H, F, and Cl) and 2,2,2-trifluoroethanol at = 310 ± 20 K. The observed reactions bring about the protonation of one of the aryl anions, which is then released as the corresponding arene, while the formed alkoxide binds to the zinc center. The protonation is faster for the more electron-rich aryl groups and shows a linear Hammett plot if the rate constant for X = NMe is discarded from the analysis.

Origin of the different reactivity of the high-valent coinage-metal complexes [RCu Me ] and [RAg Me ] (R=allyl).

High-valent tetraalkylcuprates(iii) and -argentates(iii) are key intermediates of copper- and silver-mediated C-C coupling reactions. Here, we investigate the previously reported contrasting reactivity of [RM Me ] complexes (M=Cu, Ag and R=allyl) with energy-dependent collision-induced dissociation experiments, advanced quantum-chemical calculations and kinetic computations. The gas-phase fragmentation experiments confirmed the preferred formation of the [RCuMe] anion upon collisional activation of the cuprate(iii) species, consistent with a homo-coupling reaction, whereas the silver analogue primarily yielded [AgMe ] , consistent with a cross-coupling reaction.

Phenyl argentate aggregates [AgPh] (n = 2-8): Models for the self-assembly of atom-precise polynuclear organometallics.

Electrospray ionization of phenyl argentates formed by transmetalation reactions between phenyl lithium and silver cyanide provides access to the argentate aggregates, [AgPh], which were individually mass-selected for n = 2-8 in order to generate their gas-phase Ultraviolet Photodissociation (UVPD) "action" spectra over the range 304-399 nm. A strong bathochromic shift in optical spectra was observed with increasing size/n. Theoretical calculations allowed the assignment of the experimental UVPD spectra to specific isomer(s) and provided crucial insights into the transition from the 2D to 3D structure of the metallic component with the increasing size of the complex.

Modular Ion Mobility Calibrants for Organometallic Anions Based on Tetraorganylborate Salts.

Organometallics are widely used in catalysis and synthesis. Their analysis relies heavily on mass spectrometric methods, among which traveling-wave ion mobility spectrometry (TWIMS) has gained increasing importance. Collision cross sections (CCS) obtainable by TWIMS significantly aid the structural characterization of ions in the gas phase, but for organometallics, their accuracy has been limited by the lack of appropriate calibrants.

Direct Detection of Free and Counterion-Bound Carbanions by Electrospray-Ionization Mass Spectrometry.

We propose electrospray-ionization (ESI) mass spectrometry as a robust and powerful method for the in situ analysis of carbanions. ESI mass spectrometry selectively probes the charged components of the sampled solution and, thus, is ideally suited for the detection of free carbanions. We demonstrate the potential of this method by analyzing acetonitrile solutions of 15 different carbon acids AH, whose acidities cover a range of 11.

Supramolecular Self-Assembly of β -Peptides Mediated by Janus-Type Recognition Units.

To gain mechanistic insights, natural systems with biochemical relevance are inspiring for the creation of new biomimetics with unique properties and functions. Despite progress in rational design and protein engineering, folding and intramolecular organization of individual components into supramolecular structures remains challenging and requires controlled methods. Foldamers, such as β-peptides, are structurally well defined with rigid conformations and suitable for the specific arrangement of recognition units.

Second Comes First: Switching Elementary Steps in Palladium-Catalyzed Cross-Coupling Reactions.

The electron-poor palladium(0) complex L Pd (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) reacts with Grignard reagents RMgX and organolithium compounds RLi via transmetalation to furnish the anionic organopalladates [L PdR] , as shown by negative-ion mode electrospray-ionization mass spectrometry. These palladates undergo oxidative additions of organyl halides R'X (or related S 2-type reactions) followed by further transmetalation. Gas-phase fragmentation of the resulting heteroleptic palladate(II) complexes results in the reductive elimination of the cross-coupling products RR'.

Modulation of Gas-Phase Lithium Cation Basicities by Microsolvation.

In contrast to the extensive knowledge of lithium cation affinities and basicities, the thermochemistry of microsolvated lithium cations is much less explored. Here, we determine the relative stabilities of Li(A,B) complexes, n = 2 and 3, by monitoring their gas-phase reactions with A and B substrate molecules, A/B = MeO, EtO, tetrahydrofuran, and MeCN, in a three-dimensional quadrupole-ion trap mass spectrometer. Kinetic analysis of the observed ligand displacement reactions affords equilibrium constants, which are then converted into Gibbs reaction energies.

Benzhydrylpyridinium Ions: A New Class of Thermometer Ions for the Characterization of Electrospray-Ionization Mass Spectrometers.

Previous attempts to characterize the internal energies of ions produced by electrospray ionization (ESI) have chiefly relied upon benzylpyridinium ions, R-BnPy, as thermometer ions. However, these systems are not well suited for this purpose because of their relatively high dissociation energies. Here, we propose benzhydrylpyridinium ions, R,R'-BhPy, as a new class of thermometer ions.

Formation of Transient Anionic Metal Clusters in Palladium/Diene-Catalyzed Cross-Coupling Reactions.

Despite their considerable practical value, palladium/1,3-diene-catalyzed cross-coupling reactions between Grignard reagents RMgCl and alkyl halides AlkylX remain mechanistically poorly understood. Herein, we probe the intermediates formed in these reactions by a combination of electrospray-ionization mass spectrometry, UV/Vis spectroscopy, and NMR spectroscopy. According to our results and in line with previous hypotheses, the first step of the catalytic cycle brings about transmetalation to afford organopalladate anions.

Mössbauer and mass spectrometry support for iron(ii) catalysts in enantioselective C-H activation.

A combination of electrospray-ionization mass spectrometry and Mössbauer spectroscopy was used to investigate the species generated in situ in highly enantioselective Fe/NHC-catalyzed C-H alkylations. The findings indicate an organometallic iron(ii)-NHC species to be of key relevance in the asymmetric catalysis.

Mechanisms of Cobalt/Phosphine-Catalyzed Cross-Coupling Reactions.

>The combination of CoCl with bidentate phosphines is known to catalyze challenging cross-coupling and Heck-type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray-ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low-valent cobalt complexes.

Substitution reactions of gaseous ions in a three-dimensional quadrupole ion trap.

Substitution reactions between gaseous ions and neutral substrate molecules are of ongoing high interest. To investigate these processes in a qualitative and quantitative manner, we have constructed a device, with which a defined amount of a volatile substrate can be mixed with a defined amount of helium gas and added into a three-dimensional quadrupole ion trap. From the known inner volume of the device, the known ratio n :n of the mixture, and the determined absolute partial pressure of helium in the ion trap, we can derive the partial pressure of the substrate in the ion trap and, thus, convert the directly observable pseudo-first-order rate constants of the substitution reactions into absolute bimolecular rate constants.

Large Solvent Isotope Effect Associated with the Hydrolysis of Allylindium Iodide.

Allylindium halides are valuable allylating reagents, which can even be applied in aqueous solutions. Previous H NMR spectroscopic investigations suggested that these reagents are stable for several hours in DO, whereas ESI-mass spectrometric experiments in HO pointed to much shorter lifetimes. To resolve this seeming discrepancy, the present study systematically examines the solvolysis of allylindium iodide in HO, DO, and mixtures thereof by both analytical methods.